It is common knowledge that silver halide emulsions are inherently sensitive to blue and ultraviolet radiation, while they are not sensitive to green and red radiations. For normal color photography processes, it is necessary to make these emulsions sensitive to green and red by means of spectral sensitizers (e.g. cyanide dyes) well known to the skilled in the art.
It is also well-known that ultraviolet radiation can harm the chromatic equilibrium of the photographic color images (intended as the capacity of such images to reproduce real images with the same color balance seen by human eye), because they produce a spurious UV-initiated image and therefore cause the formation of color, without any connection with the real images as seen by human eye, which does not see UV radiations, i.e. those shorter than about 420 nm. This radiation, furthermore, has destructive effects on the materials which form the photographic image when they are exposed to the light after the treatment. Color paper, for example, after having been exposed and treated, undergoes a color degradation if it does not contain a suitable ultraviolet absorber.
Compounds which protect photographic materials from the destructive effects of UV radiations efficaciously, such as hydroxy-phenylbenzotriazole compounds, have been described in various patents including U.S. Pat. Nos. 3,004,896, 3,253,921 and 4,323,633. These compounds have found large use in photography, particularly color photography.
They cannot be used, however, to prevent UV radiation from disturbing the chromatic equilibrium of photographic images.
In order to accomplish this result, in fact, compounds are to be used which absorb the ultraviolet radiation near 400 nm and not that near 420 nm, while the above mentioned hydroxy-phenylbenzotriazole compounds absorb between 300 and 370 nm.
Even if some compounds with such absorption characteristics were known, they can loose those characteristics when introduced into the layers by the normal techniques known in the art, such as the dispersion technique. An example of this is represented by compounds derived from aminoallylidenemalononitrile (such as dihexylaminoallylidenemalononitrile) whose preparation is described in Chem. Ber., Vol. 103, pages 222-235 (1970), which have shown to be compounds with a high and sharp absorption near 400 nm (and a high molar extinction coefficient), without absorbing the radiations near 420 nm. Unfortunately, they have been shown to lose their characteristics when introduced in the photographic layer according to the above-mentioned dispersion method (see U.S. Pat. No. 4,045,229). To obviate this disadvantage, the loaded polymer technique has been suggested (see BE Pat. No. 833,512) which consists in loading solid particles of a particular polymeric latex with an aminoallylidenemalonoitrile hydrophobic derivative and in mixing the so-loaded latex with the photographic layer gelatin, in which the UV absorber compound must be introduced. This technique, however, has the disadvantage that it is not suitable for obtaining consistent, reproducible results. Besides, not every polymeric latex is suitable for this aim and those that are suitable are of difficult preparation and expensive. Furthermore, the high latex/UV absorber compound ratio makes necessary the use of quantities of latex which are too high and negatively affect the physical characteristics of the layer containing it.
U.S. Pat. No. 4,307,184 has described polymeric compounds including ultraviolet absorber aminoallylidene units obtained upon copolymerization of 3-aminoallylidenemalononitrile with an ethylenically unsaturated monomer (such as an acrylic monomer). This technique appears to be more complicated than the normal dispersion techniques known in the art (see for example the above cited patents and U.S. Pat. Nos. 2,322,027; 2,533,514; 2,801,171; 2,870,012; 2,991,177; 2,739,888; 3,253,921 and British Pat. No. 1,357,372).
Briefly, the solvent dispersion technique consists in dissolving a compound in an organic solvent and then dispersing the obtained solution with an aqueous medium such as water or a gelatin in water solution. The obtained dispersion can either be directly introduced into the photographic composition before coating or can be dried to remove part or all the organic solvent prior to such introduction. In one case, high-boiling (water-immiscible) organic solvents are to be used. In the second, low-boiling organic solvents are to be used alone or mixed with high-boiling organic solvents.